I HAVE COVEREDwar, feeling the zip of bullets overhead, the heat of an explosion. I've been inside drug dens and on police stakeouts. I have watched two men die in Virginia's electric chair.

Yet nothing compares to personal encounters with people who have done something so horrible, so evil, that it defies understanding. People who can look you in the eye and describe what it was like to use a high-powered rifle to shoot a stranger in the head; how pedophilic obsession can infect, fester, destroy; how desperation can lead to animalistic rape.

LEE BOYD MALVOaided John Allen Muhammad, who orchestrated the 2002 sniper shootings in Maryland, Virginia, and the District of Columbia. Allen was put to death in 2009. What surprised me most about Malvo when I interviewed him in 2012 was his eloquence. Ten years after he and Muhammad embarked on a 23-day killing spree that took at least 10 lives, a boy had grown into a man; rage and bloodlust had morphed into contrition and, strangely, optimism.

"I see opportunity everywhere," Malvo said. All he sees for 23 hours a day — and will see every day for the rest of his life — are the walls of a tiny cell. He interacts with almost no one. Opportunity?

He has studied psychology, history, philosophy. He is into yoga and practices Eastern meditation. Malvo makes the most of a situation that he created.

Maybe more than 85,000 hours — and counting — to think about it helps. He darkened as he talked about the killings. There was no joy there, no boasting. It was horrible in the telling.

He and Muhammad scouted shooting locations, planned escapes, hid the gun near some of the outdoor sites so that, if questioned after a shooting, they would not have it with them.

One memory he said bothers him most: the death of Linda Franklin at the Home Depot in Seven Corners. Malvo told me he paced out the distance across Route 50 many times, walking across the highway to where he and Muhammad would take the shot. After the older man pulled the trigger, Malvo said, he focused on the look in the eyes of Linda Franklin's husband.

"They are penetrating," Malvo said. "It is the worst sort of pain I have ever seen in my life. His eyes... words do not possess the depth in which to fully convey that emotion and what I felt when I saw it."

Reflecting on his state of mind at the time, he said, "If there was a soul there, it was behind layers and layers and layers of darkness."

He told the tale I'd heard before: He was a wayward boy in Antigua who was taken in by Muhammad, brainwashed and transformed, like a child soldier, into a killing machine. "He told me to do something, and I did it," Malvo said.

He admitted that there must have been something very wrong with him, something that allowed the most terrible of evils to emerge. Something susceptible, easily corrupted.

Once Muhammad had secured his trust, Malvo said, "I was a lost cause."

AS WITH MALVO, my contact with Kevin Ricks began with a letter. Ricks, a high school English teacher, had been arrested in February 2010 on a charge of taking "indecent liberties" with a minor.

Ricks had moved often during the past 30 years, taking jobs at numerous schools in several states. Everywhere I called, people told me that he had suddenly left town without much explanation. Usually, the departures coincided with rumors of inappropriate relationships or apparent stalking of students.

Then 50 years old, Ricks was adamant that he had done nothing wrong. Yet every person he suggested I speak to said otherwise. They described a predator, someone who developed relationships with boys, cultivated their friends and families, then got them drunk and molested them in their sleep. He would later plead guilty to federal charges and be sentenced to 25 years in prison.

He was eager to talk. I had just returned from Denmark, where I interviewed a man who had lived with Ricks in Danville, Va., for six months in 1999 as a foreign-exchange student. I related a story the man had told me, about finding naked photographs of himself in Ricks's bedside table drawer and then confronting Ricks.

"And then..." I started, but Ricks finished the sentence: "we went out back and burned them in the barbecue."

Ricks eventually admitted pretty much anything I could confirm through my reporting. Abuse of boys in Japan. Abuse of boys in North Carolina. Abuse of boys in Virginia and Maryland and Georgia.

Ricks seemed intelligent, worldly, kind. He is a storyteller, endearing and witty — the kind of teacher you want involved in your kid's education.

When confronted with his crimes, he admitted "misbehavior" with half a dozen boys but refused to believe he had done anything to hurt anyone. He insisted that his midnight photography was artistic, a way for him to solidify relationships with teenagers without causing any lasting damage — because they were all passed out. He viewed what he did as "the least intrusive thing to do."

Ricks said his biggest failure was allowing his quest for love and intimacy to lead his "emotions to overrule logic." He maintained that his abuse of teenage boys, although morally and legally wrong, was spiritually justified because he loved the boys and did not mean to hurt them.

"I don't think it can be attributed to a criminal mind," Ricks said. "My behaviors in this case and in several cases are illegal, and I don't disagree with the law. But my set of beliefs and my perception are misguided at worst. The road to hell is paved with good intentions, and I'm certainly on it."

Ricks wavered between understanding and denial. "I don't believe that I'm a danger or threat to anyone," he said at one point. But later he contradicted himself: "I have no control with minors. I should not be around minors."

He admitted an "infatuation" with young boys. He became the teacher they all wanted, someone who was easygoing, had few rules, and tried to relate to students.

I knew he had a teenage relative, so I asked him what he would think if that relative came to him and described a relationship with a 50-year-old English teacher, if he learned that the teen and the teacher had been drinking together, and if he saw incriminating photographs of what the teacher had done to the unconscious relative.

Ricks wiped his eyes. Tears.

"It's reprehensible," he said. "Reprehensible."

Ricks straightened up. "But I loved them," he said.

WHEN AARON THOMAS was arrested in New Haven, Conn., in March 2011, police were certain that he was the infamous East Coast Rapist, responsible for at least a dozen attacks on women in Maryland, Virginia, Connecticut, and Rhode Island dating back to the 1990s. His DNA matched evidence recovered at numerous crime scenes, and he immediately confessed to the attacks, according to police.

I connected with Thomas in April 2012. He began with a plea for help, saying he was living with a demon inside him, a second personality he called "Erwin." Erwin, he said, was the rapist. Aaron, he said, was a good person, a family man, someone who helped the elderly and had love for everyone.

"Erwin would show up, and things would happen in the middle of the night," Thomas said. "He only comes around when I'm lonely."

He sounded afflicted, damaged. Authorities had reported that Thomas, then 40, was suicidal and had tried to kill himself; Thomas told me that it was Erwin.

At one point, he told me he had cut his genitals in an attempt to get rid of the evil, a wound that jail officials had not discovered. With his permission, I reported the injury to jail authorities, and they responded and treated him.

We traced his life from his childhood as the son of a D.C. police officer and a career Geico employee to his years after high school as a vagrant who sometimes lived in abandoned cars, buses, and a vacant pet store; his rowdy youth, abuse at the hands of his father, hints of severe behavioral problems, and a possible mental disorder.

But it was his experiences on the street — getting shot in D.C., living homeless — that he said led to Erwin's existence. He stopped caring, got sexual urges, and struck out in the night after women.

After I spoke to experts, it seemed unusual that someone with multiple personalities would experience interaction between them. Yet Erwin was an open book to Thomas. He saw, experienced, and regretted everything that he claimed Erwin had done.

But everything he told me about his personal history checked out. He was, in fact, shot in D.C. The abandoned pet store — which later burned down, according to its owner — was indeed where Thomas said it was at the time he squatted there. Family members confirmed dates, stories he told.

As I pressed, the Erwin story fell apart. In early June, Thomas admitted that he had made Erwin up.

The real Thomas was, in many ways, scarier than the devil personality he had created. He had little explanation for what had happened and had foggy memories, at best, of the attacks. He said that he basically saw women as prey, not as people, and that there are many predators out there just like him.

"They were objects," Thomas said. "Whoever came down the street, an object... It's awful."

I asked him if he understood how dangerous he had been, if he understood that his victims were daughters, sisters, mothers, real people — if he would want revenge against a person who did these things to someone he knew.

"I never hurt anyone," he said, noting that he never beat, shot, or stabbed any of his victims. The weapons he used were often surprise and threat. He didn't seem to see rape as violence. Several of his victims, unsurprisingly, told me it was an extremely violent experience.

Thomas pleaded guilty to multiple rape and abduction charges and faces the possibility of several life sentences.

PEOPLE OFTEN ASKme how I can sit in a room with a killer or a rapist or a molester, and whether I'm giving them a chance to explain away their crimes. Of course I know that some of what they say could be self-serving, maybe outright lies. But I challenge them, verify what they say to the extent possible.

Ricks was far more likable than most people might imagine. I came to understand that his personality, his apparent trustworthiness, was his most effective weapon.

Thomas, when speaking as himself, said he barely understood why the rapes happened, but even more shocking to me was his ability to see those acts as nonviolent.

Malvo, unlike Thomas and Ricks, owns his crimes. They are a part of him — though one that he is trying to forget and that he hopes everyone else will, too. "I did monstrous things," he said.